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Date:   Fri, 1 Dec 2023 17:17:35 +0200
From:   Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>
To:     Pekka Paalanen <ppaalanen@gmail.com>
Cc:     Jernej Skrabec <jernej.skrabec@gmail.com>,
        Emma Anholt <emma@anholt.net>,
        Jonathan Corbet <corbet@lwn.net>,
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        Samuel Holland <samuel@sholland.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        Sandy Huang <hjc@rock-chips.com>,
        Hans Verkuil <hverkuil@xs4all.nl>,
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        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
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        linux-media@vger.kernel.org
Subject: Re: [PATCH v4 05/45] drm/connector: Check drm_connector_init
 pointers arguments
Message-ID: <ZWn5D387DYmsh1sa@intel.com>
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On Wed, Nov 29, 2023 at 12:25:37PM +0200, Ville Syrj?l? wrote:
> On Wed, Nov 29, 2023 at 12:12:59PM +0200, Pekka Paalanen wrote:
> > On Tue, 28 Nov 2023 15:49:08 +0200
> > Ville Syrj?l? <ville.syrjala@linux.intel.com> wrote:
> > 
> > > Should we perhaps start to use the (arguably hideous)
> > >  - void f(struct foo *bar)
> > >  + void f(struct foo bar[static 1])
> > > syntax to tell the compiler we don't accept NULL pointers?
> > > 
> > > Hmm. Apparently that has the same problem as using any
> > > other kind of array syntax in the prototype. That is,
> > > the compiler demands to know the definition of 'struct foo'
> > > even though we're passing in effectively a pointer. Sigh.
> > 
> > 
> > __attribute__((nonnull)) ?
> 
> I guess that would work, though the syntax is horrible when
> you need to flag specific arguments.

I played around with this a bit (blindly cocci'd tons of
drm and i915 function declarations with the nonnull attribute)
and it's somewhat underwhelming unfortunately.

It will trip only if the compiler is 100% sure you're passing
in a NULL. There is no way to eg. tell the compiler that a
function can return a NULL and thus anything coming from it
should be checked by the caller before passing it on to
something with the nonnull attribute. And I suppose error
pointers would also screw that idea over anyway.

Additionally the NULL device checks being being done in 
the drm_err/dbg macros trip this up left right and center.
And hiding that check inside a function (instead of having
it in the macro) is also ruined by the fact that we apparently
pass different types of pointers to these macros :( Generics
could be used to sort out that type mess I suppose, or the
code that passes the wrong type (DSI code at least) should
just be changed to not do that. But not sure if there's enough
benefit to warrant the work.

-- 
Ville Syrj?l?
Intel